The term "saturated liquid" is used in this application to describe a liquid at its exact boiling point for a corresponding vapor pressure. A common situation which involves pumping saturated liquids occurs where the gas is the vapor phase of the liquid substance. Either a decrease of the pressure or an increase in the temperature will vaporize some of the liquid until a new boiling point equilibrium at a new vapor pressure is reached. This situation is found in the storage and transportation of cryogenic liquids, which exist in gaseous phase at normal ambient temperatures and pressures, but can be stored and transported in well-insulated low temperature containers in their liquid state, thus drastically reducing their volume without need for increasing the pressure. Liquid elements such as hydrogen, oxygen or nitrogen and liquified hydrocarbons such as methane or natural gas are common examples of cryogenic liquids.
Reciprocating pumps are frequently used to move such cryogenic fluids between containers or to any other point of use. With the conventional pumps of the prior art, it has been found necessary to maintain a net positive suction head (NPSH) to prevent cavitation within the pump. Net positive suction head (NPSH) is the required additional pressure above the saturation vapor pressure of the liquid at any given temperature. Cavitation is the formation of vapor-filled cavities within the liquid caused in areas of the pump where the pressure of the moving liquid drops below the saturation vapor pressure. It typically occurs at the inlet valve of the prior art pumps, as the initial opening of the valve allows liquid to be drawn into an evacuated chamber. The pressure drop associated with the accelerated liquid produces vapor cavities, which become trapped in the cylinder below the piston. As the piston moves down on the compression stroke, the liquid pressure is increased above its saturation vapor pressure and the vapor cavities instantly condense back into liquid. The collapsing cavities generally cause significant shock, vibration, noise, and rapid erosion of metal surfaces, all of which can damage the pump.
For brevity purposes, the remainder of this disclosure will refer only to saturated liquid or to a particular saturated liquid such as liquified natural gas. It should be understood that the invention likewise applies to near saturated liquids, which in the context of this application means a liquid sufficiently near saturation that cavitation is a potential problem when it is being pumped.